Radio receiver



March 23, 1943- J. D. HOLLAND 2,314,785

' RADIO RECEIVER Filed Feb. 27, 1942 Patented Mar. 23, 1943 UNiTsD STATES PATENT creme RADIO lRECEIVER John Douglas Holland, London, England, assignor to International Standard Electric Corporation,

New York, N. Y.

Application February 27, 1942, Serial No. 432,589 En Great Britain February 7, 1941 2 Claims. (Cl. Z50-20) lo inject the local heterodyne or C. W. oscillation v at a point in Jthe intermediate frequency amplifier, usually the last stage. This arrangement will cause the automatic gain control to come into operation and bias back the controlled valves.'

This desensitising of the receiver or amplifier can be partially avoided by the well known expedient of supplying a delay voltage so that the controlled valves are not affected by injection voltage due to the local C. W. oscillator. It will then be found, however, that the delay voltage will not be of the correct value for the reception of modulated signals with the local C. W. oscillator switched off. Alternatively, as acompromise, the strength of the local C. W. oscillator may be reduced, but it will then be found that it may be easily pulled into step or zero beat by a strong signal. Furthermore, the theoretically desirable condition that the local C. W. oscillator should be stronger than the signal will not always be met. Moreover, the strength of the local oscillator will also determine the strength of the harmonics it produces. These harmonics may occur at a frequency where signal frequency reception is desired.

It is the object of the present invention to overcome these difficulties and according thereto a circuit arrangement for the reception of C. W. signals by means of a superheterodyne receiver employing automatic gain control is characterised in that the local continuous oscillations beating with the intermediate frequency are not injected directly into the intermediate frequency amplifier provided with automatic gain control, but separate electric circuit paths are provided after the intermediate frequency amplifier having automatic gain control for reception of continuous Wave signals and of modulated signals.

According to another aspect of the invention in circuit arrangements for the reception of continuous wave signals by means of a superheterodyne receiver employing automatic gain control,

the continuous oscillations for beating with the intermediate frequency are combined with the in` termediate frequency in a stage separate from the intermediate frequency amplifier having automatic gain control and having a minimum of coupling with the said intermediate frequency 'Y amplifying stage having the automatic gain con-'fl trol.

the following description taken in conjunction with the accompanying drawing which shows by way of example, a circuit arrangement according to the invention incorporated in a system for re" ceiving speech signals as well as continuous wave signals and in which a'low frequency amplifying stage is common tothe' reception of both signals." In the drawing there is shown a heXode-triode v thermionic valve'Vi having a'cathode K (shown as indirectly heated) and a rst grid g1 common to the hexode and triode electrode combinations.v

The triocle portion is completed by an anode AI,k

and the hexode portion is completed by grids g3, g4 and anode A2.

sistancel Rl and capacity C4 are provided in Well known manner.

On the hexode side of valve Vl, the intermediate frequency continuous wave signals are applied between the terminal A and earth. The lead from terminal A to earth includes an inductance LI which is coupled to another inductance L2 connected between grid g3 which functions as a control grid, and the cathode K via resistance RI. L2 is a low impedance coupling coil, and LI may be part of a tuned circuit at intermediate frequency. The anode A2 receives its high tension voltage Via terminal H.T.} via antiresonant circuit L3, Cl tuned to the difference frequency obtained between the intermediate frequency signal and the local beat oscillator, and the grids g2, g4 receive a positive voltage from H.T.-} via resistance R4. The anode AI of the local heterod'yne oscillator receives its high tension voltage also from II.T.+ Via an adjustable tap on the inductance L4. This lead includes a switch S2 so that when reception of modulated signals is desired, the local oscillator can be put out of action. The output from the valve VI is applied via a switch Si and resistance-capacity coupling C5, R2 to a low frequency amplifier V2 shown as a triode thermionic valve,

The invention will be better understood-from the grid g of V2 being connected to an adjustable tap on R2 which thus acts as a volume control. The switch SI is a change-over switch, one terminal C of which is connected to the output circuit of valvev VI as already stated and the other terminal D is connected at B to the output from the rectifier stage (not shown) of a receiver for speech reception. A pair of headphones H or other signal indicating device is connected in the output circuit of the low frequency ampliiier V2.

For continuous wave signal reception, the oscillator is tuned to a frequency which differs from v the intermediate frequency received by some 1000 cycles. Hence when the switch Si is closed on terminal C as shown and switch S2 is closed the oscillations from the grid g3 and the oscillations produced by the local oscillator (Al, g1, K) will amplifier system and the local C. W. oscillator is negligible as only a small portion of the total intermediate frequency voltage is applied between g3 and ground, and the circuits of the C. W. oscillator comprising L4C3 and the triode portion of VI outside the valve are separated from the circuits of the hexode portion outside the valve. Hence the local heterodyne oscillator can be set for optimum conversion gain in the valve VI without affecting the delay voltage that is associated with the rectification diodes (not shown) that are part of the intermediate frequency amplifier and pulling into step of the local oscillator by an incoming signal is remote due to the screening within the valve and also by using electronic coupling between the C. W. oscillator and the circuit of the signal at intermediate frequency.

Although the oscillator is only some 1000 cycles per second different from the intermediate frequency of the signals, coil L2 is effectively untuned and hence no positive feedback can result between L2 and L4.

By critical dimensioning of CI and L3 a good note lter effect can be obtained, the damping of the lter being small due to the input impedance, namely, that of the hexode portion of VI, and the output impedance C5, R2 being of high value. If a note filtering effect is not required, CI, L3 may be replaced by a resistance of suitable value.

Another advantage of the circuit arrangement described is that by providing a two channel path, namely, one for continuous wave signals and one for speech signals, the circuit design for both is simplified.

Whilst in the example given, a heXode-triode valve has been described, any multigrid valve which functions by virtue of electronic mixing may be employed. Y

What is claimed is:

1. In a superheterodyne receiver of the type having an intermediate frequency wave circuit including an intermediate frequency amplifier adapted for the reception of continuous or speech modulated waves having automatic volume control means connected therewith, and a local oscillator for supplying a continuous wave to beat with the amplied continuous wave from said intermediate frequency amplifier, the combina-tion including electronic means for producing a beat wave in response to waves from said intermediate frequency amplier and said oscillator, said oscillator being velectronically coupled with said beat producing means.

2. The combination according to claim 1, in which said beat producing means and said oscillator each include cathode, anode, and grid electrodes, said grid and anode of said beat producing means being separate from said oscillator grid` and anode, said oscillator cathode and grid being common to said beat producing means.

JOHN DOGLAS HOLLAND. 

